Inside the Race to Secure FAA Aviation Airwaves

The skies are becoming dangerously crowded: not with planes, but with interference and jamming disrupting many of the 30,000 radio frequencies that civil aviation relies upon. In previous decades, designers of aviation weather, navigation, and voice communication systems prioritized convenience over security, leaving today’s aircraft vulnerable to both unwitting interference and malicious jamming and spoofing, especially near conflict zones.

Driven by growing threats and a critical U.S. Government Accountability Office report, the Federal Aviation Administration (FAA) is now seeking ambitious monitoring and detection systems to protect its spectrum nationwide.

At an industry engagement day in mid-October, the FAA’s Vice President of Technical Operations, Dan Murphy, laid out the problem: “Right now, somewhere in the National Airspace System, a signal that aviation depends upon is being jammed, being spoofed, or just being degraded by something else happening in the spectrum.”

If air traffic operations are affected, the FAA will search for the source, a process that can take anywhere from a few days to a month or more. In the case of intermittent signals, the culprit might never be found. “This model is no longer working for us,” said Murphy. “It was designed for a quieter world in the aviation spectrum that no longer exists. We must be able to find the source of spectrum interference in seconds or minutes, not days or weeks.”

A flying start

Although some of the industry day proceedings were classified, the list of those attending suggests which technologies the FAA may be considering. The technologies on offer could enable the FAA to detect, analyze, and geolocate radio interference on an unprecedented geographic scale, at unprecedented speed.

The agency already uses data generated by aircraft in flight. The vast majority of aircraft flying today are required to use an ADS-B transponder, a safety system that broadcasts their ID and position every second or so. Nearby planes and ground stations can receive that data and use it to avoid collisions.

A global navigation satellite signal strength plot shows jammed signals.ESA

Among the data is a signal that reflects the plane’s GPS receiver’s confidence in its location: a high number means it’s receiving a good GPS signal, a lower number might suggest interference. Several organizations, including Stanford University, aggregate ADS-B data from thousands of aircraft to plot helpful near-real-time maps of interference events.

But ADS-B surveillance has its limitations, says Todd Humphreys, director of the Radionavigation Laboratory at the University of Texas at Austin: “The data remain quite noisy, because you’re dealing with different receiver platforms and different aircraft. You need a lot of traffic through an area to be confident about interference, and of course you don’t have coverage over areas where there aren’t any flights.”

Listening from space

One solution is to get a wider view of the problem, from space. Several commercial operators of low earth orbit (LEO) spacecraft, including HawkEye 360 and Spire Global are already listening to vast swathes of the radio spectrum.

Spire Global tracks navigation data accuracy for aviation purposes.Spire Global

For aviation customers, Spire can use ADS-B data to identify a zone of possible jamming, then task a pair of geolocation satellites to the area. Two satellites simultaneously record the same stretch of spectrum, relying on differences in the signals’ arrival times and Doppler shifts to estimate a transmitter’s location.

“In a recent analysis of recordings, we estimated the location of GPS jammers from a single satellite pass,” says Bryan Cuthbertson, Spire’s senior manager for Space Reconnaissance. Spire is also working on a European Space Agency (ESA) project called Eurialo, that uses ADS-B and other aircraft transmissions to precisely locate aircraft independent of GPS and ground-based radar. Like Spire’s jamming detection, it relies on the time difference of arrival of signals. “Satellite plus ADS-B seems a pretty compelling combination,” says Humphreys. However, there are not enough radio frequency-capturing satellites today to provide continuous coverage and geolocation across the entire U.S. The FAA would either have to accept delays as satellites visit different parts of the country in turn, or wait or pay for companies to launch more.

Ground truth

Another approach is to leverage assets already in place on the surface. Most modern cellphone towers have one or more GPS receivers, which collect timing signals to synchronize transmissions, as well as navigation data to boost location accuracy in cellphones in their service area. Just like the receivers in aircraft, those units can assess nearby interference. Austrian start-up Dimetor is now working to gather that data and make it available to aviation, assisted by another ESA project called NAVISP.

“All of a sudden, you have access to a network of tens of thousands of sensors… so a pilot can know if they’re jammed or not 20 miles [32 km] ahead,” says Dimetor’s CEO, Thomas Neubauer. “We can provide essentially real-time situational awareness of GPS interference in the lower airspace.”

This interference detection method would be concentrated where safety is most critical, near airports and population centers, but be less useful in more rural areas or over the open ocean. Neubauer believes such a network could become particularly useful as low altitude delivery drones and air taxi services come online. Since it is a software-based solution, it should be relatively cheap to deploy.

Synthesizing data sources from terrestrial, airborne, and space-based sensors, should be technically and mathematically straightforward, says Humphreys: “The hard part is certifying that your system will never indicate a cell red when it is actually green or vice versa. If the FAA wants to treat this like an aviation-certified system that is almost never wrong, you’re looking at years of effort and many, many millions of dollars of expense.”

There’s another problem. “Denial of service,” says Humphreys. “If the FAA were to begin to trust this map so much that they route traffic around an area, that would be something for a malicious practitioner to play with. Having to check the provenance of the data all the way to their source and certify each of those listening posts could really increase the cost.”

Mark Carmouche, a group manager for the FAA’s Spectrum Engineering division, admitted that the agency does not have even a rough order of magnitude for the price-tag of the technology it wants. “No clue,” he said. “But everybody’s seen the conflicts abroad and seen what happens with jamming and spoofing to confuse an operator or confuse our perception of what’s real. We need to learn how to protect against that.”